In these years, there is a growing demand for display devices used, for example, for a flat-screen television, a personal computer, and a mobile terminal. Such display devices are generally implemented as flat displays shaped like flat plates. Examples of flat displays include a plasma display panel (PDP) employing plasma display elements as display media, a liquid crystal display (LCD) employing liquid crystal display elements as display media, and an organic electroluminescent (EL) display employing organic EL display elements as display media. Also, display devices employing other types of display elements as display media are being developed.
As a backboard of a flat display, an array substrate is widely used. An array substrate includes an array of drive elements (switching elements) for independently driving display elements in respective pixel areas.
For the drive elements (switching elements), thin-film transistors (TFT) are popularly used.
Meanwhile, a multilayer process based on photolithography is widely used to manufacture an array substrate including an array of drive elements (switching elements) such as TFTs.
In photolithography, a conducting layer (film) as a material of scan lines (gate lines) is formed, for example, by vapor deposition, a positive resist is applied onto the conducting layer, and portions of the positive resist other than the portions corresponding to the scan lines are exposed by an exposure method such as stepper exposure, proximity exposure, or contact exposure. With the contact exposure, the production yield may be reduced because a photo mask is placed in contact with a resist. Therefore, in these days, the stepper exposure is preferred over the contact exposure. As still another example, scan exposure employing the principle of stepper exposure is also used to meet the recent increase in the size of display panels. After the exposure, developer (developing agent) is supplied to the surface of the array substrate to remove only the exposed portions of the positive resist. Next, portions of the conducting layer where the positive resist is not present are removed by etching such as wet etching or dry etching. Then, the remaining positive resist and other foreign substances are removed using a remover to form the scan lines. After forming the scan lines, a scan line insulating film, a silicon film made of, for example, amorphous silicon and used as a semiconductor layer, and a silicon nitride film are formed by repeating steps including film deposition such as chemical vapor deposition (CVD), positive resist application, exposure, development, etching, and removal in a manner similar to the above scan line forming process. Then, signal lines (source lines) are formed by performing steps including film deposition, positive resist application, exposure, development, etching, and removal in a manner similar to the scan line forming process. Normally, data lines (drain electrodes) are also formed at the same time as forming the signal lines.
The above multilayer process applies to inorganic thin-film transistors that use a silicon film as a semiconductor layer of drive elements (switching elements). When manufacturing organic thin-film transistors that use an organic semiconductor film instead of a silicon film as a semiconductor layer, no silicon film (e.g., amorphous silicon film) is formed during the film deposition step (e.g., CVD). In this case, only a scan line insulating film is formed during the film deposition step such as CVD, and an organic semiconductor film is formed between signal lines and drain electrodes (i.e., in channel regions) above the scan lines by, for example, printing or photolithography.
Then, pixel electrodes used to drive display elements by the drive elements (switching elements) and drain wires for connecting the pixel electrodes and the drain electrodes are formed. In a case where liquid crystal display elements are employed; pixel electrodes and drain wires may be formed at the same time. Generally, the same highly-conductive material is used for the source electrodes and the drain electrodes of transistors. For the pixel electrodes that have to be optically transparent, a transparent electrode material is used. Since transparent electrode materials are normally not highly conductive, a material other than the transparent electrode material used for the pixel electrodes is used for the drain electrodes of transistors. Meanwhile, in the case of a reflective display device such as a reflective LCD, signal lines, drain electrodes, and pixel electrodes can be formed at the same time using the same highly-conductive material. However, in the case of a transmissive display device requiring a high numerical aperture, a transparent electrode material different from the material of drain electrodes is used for pixel electrodes. Therefore, in a manufacturing process of an array substrate of a transmissive display device, a nonconductive film (insulating film) with high transparency is formed between drain electrodes and pixel electrodes, and holes (openings) are formed in the nonconductive film (insulating film) to accommodate drain wires for connecting the pixel electrodes and the drain electrodes. A similar manufacturing process is used to manufacture an array substrate used as a backboard of an organic EL display device. Also, there are display devices using polysilicon for the semiconductor layer of switching elements (transistors).
In manufacturing large-area, high-density display devices by a multilayer process as described above, defects such as broken lines (wires) and leakage between lines (wires) may be generated and as a result, the production yield may be reduced. Various methods have been proposed to repair (or fix) such a defect in a substrate instead of discarding the substrate and thereby to improve the production yield. For example, patent document 1 discloses a technology for repairing a broken line by fusing broken ends with a laser beam. Also, there is a known method where a liquid is discharged (e.g., using a micro dispenser or an inkjet device) to draw a line and thereby to repair a broken line.
Further, patent document 2 discloses a method used when a broken line cannot be sufficiently repaired in an active area. In the disclosed method, backup lines are formed outside of the active area to transmit a signal from an input end to a non-input end.
However, related-art methods of repairing a broken line in a display device have problems as described below.
With a method employing laser CVD, the equipment for laser CVD is very expensive.
With the method where a line is drawn by discharging a liquid using a micro dispenser or an inkjet device, a broken line often cannot be repaired or it is difficult to detect a failure in repairing the broken line on the spot.
In the method disclosed in patent document 2, backup lines intersecting signal lines or scan lines are formed in advance. When a break is found in a signal or scan line, the backup lines and the signal or scan line are connected by fusing at the intersections to repair the break. With this method, leakage occurs at the intersections (or crossing points) that are not used for repairing broken lines and as a result, cross capacitance increases. As the resolution of a display device increases, the number of necessary backup lines increases. This in turn increases the number of leakage points and results in further increase in the cross capacitance.    [Patent document 1] Japanese Patent Application Publication No. 2004-198718    [Patent document 2] Japanese Patent Application Publication No. 5-127191